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| United States Patent |
5,171,940
|
|
Vallauri
|
December 15, 1992
|
Expanded cable joint elastic sleeves with permissible residual
deformation after storage
Abstract
A storable covering element for electric cable joints which includes a
tubular support on which a multi-layer sleeve is mounted in radially
stretched condition, a cable joint with such a sleeve and a sleeve for
such use. The sleeve has an inner layer of cross-linked polymeric material
which has a temporary residual deformation after storage for at least 24
months at room temperature which is less than such deformation of the
layers outwardly thereof and preferably has a modulus of elasticity
greater than the modulus of elasticity of the layers outwardly thereof.
The outer layers are also made of cross-linked polymeric material, and
each layer is bonded to the adjacent layer so that the inner layer will
pull the outer layers radially inwardly when the sleeve is applied to a
cable joint.
| Inventors:
|
Vallauri; Ubaldo (Monza, IT)
|
| Assignee:
|
Societa' Cavi Pirelli S.p.A. (Milan, IT)
|
| Appl. No.:
|
589073 |
| Filed:
|
September 27, 1990 |
Foreign Application Priority Data
| Oct 11, 1989[IT] | 21979 A/89 |
| Current U.S. Class: |
174/73.1; 174/84R; 174/93; 174/DIG.8 |
| Intern'l Class: |
H02G 015/08; H02G 015/18 |
| Field of Search: |
174/73.1,93,88 C,84 R,DIG. 8
29/235,450
|
References Cited
U.S. Patent Documents
| 3515798 | Jun., 1970 | Sievert | 174/135.
|
| 3816640 | Jun., 1974 | Varner | 174/73.
|
| 3990479 | Nov., 1976 | Stine et al. | 138/125.
|
| 3992567 | Nov., 1976 | Malia | 174/73.
|
| 4079189 | Mar., 1978 | Troccoli | 174/73.
|
| 4238639 | Dec., 1980 | Palmieri | 174/73.
|
| 4304616 | Dec., 1981 | Richardson | 174/73.
|
| 4314093 | Feb., 1982 | Eldridge et al. | 174/73.
|
| 4363842 | Dec., 1982 | Nelson | 174/73.
|
| 4383131 | May., 1983 | Clabburn | 174/73.
|
| 4487994 | Dec., 1984 | Bahder | 174/73.
|
| 4503105 | Mar., 1985 | Tomioka | 174/135.
|
| 4613533 | Sep., 1986 | Loomis et al. | 428/36.
|
| 4868967 | Sep., 1989 | Holt et al. | 174/135.
|
| Foreign Patent Documents |
| 0149032 | Jul., 1985 | EP.
| |
| 0379056 | Jul., 1990 | EP.
| |
| 0415082 | Mar., 1991 | EP.
| |
| 3001158 | Jul., 1980 | DE.
| |
| 0049588 | Apr., 1979 | JP.
| |
| 1294665 | Nov., 1972 | GB.
| |
| 1337951 | Nov., 1973 | GB.
| |
| 2046032 | Nov., 1980 | GB.
| |
| 2183935 | Jun., 1987 | GB | 174/73.
|
Primary Examiner: Picard; Leo P.
Assistant Examiner: Sough; Hyung S.
Attorney, Agent or Firm: Brooks Haidt Haffner & Delahunty
Parent Case Text
RELATED APPLICATIONS
This application is a continuation-in-part of copending applications Ser.
No. 07/464,370 filed Jan. 12, 1990, entitled Multi-Layer Elastic Sleeves
For Electric Power Cable Joints and Joints Therewith, Ser. No. 07/508,783
filed Apr. 12, 1990, entitled Cable Joint Coverings, Devices for Applying
Such Coverings and Joints Obtained Therewith and Ser. No. 07/560,359 filed
Jul. 31, 1990, entitled Expanded Cable Joint Elastic Sleeves with
Permissible Residual Deformation After Storage, all assigned to the
assignee of the present application.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A storable tubular element for covering electrical cable joints and
adapted to be applied to a group of cables of several different outer
diameters, said element comprising:
a tubular support having an internal diameter at least equal to the largest
diameter of the cable in said group and having a predetermined outer
diameter;
an elastic sleeve removably mounted on said support in radially stretched
condition, said sleeve having an inner diameter in its unstretched
condition less than said predetermined outer diameter of said support and
less than the diameter of the cable in said group having the smallest
diameter and said support having sufficiently rigidity to maintain said
sleeve in said radially stretched condition, said sleeve comprising an
innermost layer of cross-linked polymeric material and at least one outer
layer of cross-linked polymeric material coaxial with said innermost
layer, each layer being bonded to the next adjacent layer and each layer
reducing in internal diameter to a diameter smaller than the stretched
diameter thereof upon removal of said tubular support by reason of the
elasticity thereof and without heating thereof, at least one of the layers
around said innermost layer being electrically insulating and the material
of said innermost layer, in the cross-linked state, having a temporary
residual deformation less than the temporary residual deformation of the
material of the layers outwardly thereof after said sleeve has been
mounted on said support in stretched condition for at least twenty-four
months at room temperature and upon removal of said support and the
elastic modulus and radial thickness of the material of said innermost
layer being selected to cause said innermost layer, upon removal of said
support, to apply a predetermined radially inward pressure on the cables
in said group to which the sleeve is applied including the cable in said
group having the smallest diameter.
2. A storable tubular element as set forth in claim 1 wherein said
temporary residual deformation of said innermost layer is less than 15%
when said innermost layer has been maintained under an expansion of 50%.
3. A storable tubular element as set forth in claim 2 wherein said
temporary residual deformation of said innermost layer is less than 15%
when said innermost layer has been maintained under an expansion of 50%
for at least 40 days at 65.degree. C.
4. A storable tubular element as set forth in claim 1 wherein said
innermost layer has a modulus of elasticity at least as great as the
modulus of elasticity of the layers outwardly thereof.
5. A storable tubular element as set forth in claim 4 wherein the radial
thickness of said innermost layer is less than 25% of the radial thickness
of said sleeve and said innermost layer has a modulus of elasticity at
least 1.5 times the modulus of elasticity of the layers outwardly thereof.
6. A storable tubular element as set forth in claim 4 wherein the modulus
of elasticity of said innermost layer is in the range from 0.5 MPa to 10
MPa and the modulus of elasticity of the layers outwardly thereof is in
the range from 0.03 MPa to 6 MPa.
7. A storable tubular element as set forth in claim 4 wherein the modulus
of elasticity of said innermost layer is in the range from 1 MPa to 5 MPa
and the modulus of elasticity of the layers outwardly thereof is in the
range from 0.5 MPa to 3 MPa.
8. A storable tubular element as set forth in claim 1 wherein each layer is
made of an elastomeric material having a chemical affinity with the
elastomeric material of the layer adjacent thereto and the layers are
co-extruded.
9. A storable tubular element as set forth in claim 1 wherein each layer is
made of an elastomeric material having a chemical affinity with the
elastomeric material of the layer adjacent thereto and the layers are
jointly cross-linked.
10. A storable tubular element as in claim 1 wherein said predetermined
radially inward pressure is at least 0.1 MPa.
11. An electric cable joint between two cables, each cable having a
conductor encircled by insulation and the conductor of one cable being
electrically and mechanically connected at its end to the end of the
conductor of the other cable, and an elastic sleeve encircling the
connected conductor ends and portions of the insulation of each cable,
said sleeve comprising:
an innermost layer of cross-linked polymeric material and at least one
outer layer of cross-linked polymeric material coaxial with said innermost
layer, each layer being bonded to the next adjacent layer and at least one
of the layers around said innermost layer being electrically insulating,
said innermost layer and the layers therearound being radially stretched
condition with said innermost layer applying a predetermined radially
inward pressure to the insulation of each cable and the material of said
innermost layer, in the cross-linked state, having a temporary residual
deformation less than the temporary residual deformation of the material
of the layers outwardly thereof after the materials of the layers have
been in stretched condition for at least 24 months at room temperature.
12. A joint as set forth in claim 11 wherein said temporary residual
deformation of said innermost layer is less than 15% when said innermost
layer has been maintained under an expansion of 50%.
13. A joint as set forth in claim 12 wherein said temporary residual
deformation said innermost layer is less than 15% when said innermost
layer has been maintained under an expansion of 50% for at least 40 days
at 65.degree. C.
14. A joint as set forth in claim 11 wherein said innermost layer has a
modulus of elasticity at least as great as the modulus of elasticity of
the layers outwardly thereof.
15. A joint as set forth in claim 14 wherein the radial thickness of said
innermost layer is less than 25% of the radial thickness of said sleeve
and said innermost layer has a modulus of elasticity at least 1.5 times
the modulus of elasticity of the layers outwardly thereof.
16. A joint as set forth in claim 14 wherein the modulus of elasticity of
said innermost layer is in the range from 0.5 MPa to 10 MPa and the
modulus of elasticity of the layers outwardly thereof is in the range from
0.03 MPa to 6 MPa.
17. A joint as set forth in claim 14 wherein the modulus of elasticity of
said innermost layer is in the range of 1 MPa to 5 MPa and the modulus of
elasticity of the layers outwardly thereof is in the range from 0.5 MPa to
6 MPa.
18. A joint as set forth in claim 11 wherein each layer is made of an
elastomeric material having a chemical affinity with the elastomeric
material of the layer adjacent thereto and the layers are co-extruded.
19. A joint as set forth in claim 11 wherein each layer is made of an
elastomeric material having a chemical affinity with the elastomeric
material of the layer adjacent thereto and the layers are jointly
cross-linked.
20. A storable tubular element as in claim 11 wherein said predetermined
radially inward pressure is at least 0.1 MPa.
21. An elastic sleeve for covering an electric cable joint, said sleeve
comprising:
an innermost layer of cross-linked polymeric material;
at least one outer layer of cross-linked polymeric material coaxial with
said innermost layer, each layer being bonded to the next adjacent layer
and at least one of the layers around said innermost layer being
electrically insulating, the material of said innermost layer, in the
cross-linked state, having a temporary residual deformation less than the
temporary residual deformation of the material of the layers outwardly
thereof after the material of the layers have been in stretched condition
for at least 24 months at room temperature.
22. A sleeve as set forth in claim 21 wherein said temporary residual
deformation of said innermost layer is less than 15% when said innermost
layer has been maintained under an expansion of 50%.
23. A sleeve as set forth in claim 22 wherein said temporary residual
deformation of said innermost layer is less than 15% when said innermost
layer has been maintained under an expansion of 50% for at least 40 days
at 65.degree. C.
24. A sleeve as set forth in claim 21 wherein said innermost layer has a
modulus of elasticity at least as great as the modulus of elasticity of
the layers outwardly thereof.
25. A sleeve as set forth in claim 24 wherein the radial thickness of said
innermost layer is less than 25% of the radial thickness of said sleeve
and said innermost layer has a modulus of elasticity at least 1.5 times
the modulus of elasticity of the layers outwardly thereof.
26. A sleeve as, set forth in claim 24 wherein the modulus of elasticity of
said innermost layer is in the range from 0.5 MPa to 10 MPa and the
modulus of elasticity of the layers outwardly thereof is in the range from
0.03 MPa to 6 MPa.
27. A sleeve as set forth in claim 24 wherein the modulus of elasticity of
said innermost layer is in the range from 1 MPa to 5 MPa and the modulus
of elasticity of the layers outwardly thereof is in the range from 0.5 MPa
to 3 MPa.
28. A sleeve as set forth in claim 21 wherein each layer is made of an
elastomeric material having a chemical affinity with the elastomeric
material of the layer adjacent thereto and the layers are co-extruded.
29. A sleeve as set forth in claim 21 wherein each layer is made of an
elastomeric material having a chemical affinity with the elastomeric
material of the layer adjacent thereto and the layers are jointly
cross-linked.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a covering sleeve for cable joints made of
compounds of cross-linked polymeric material which can be applied to
several different cables having different outer diameters.
In order to provide a junction between electric cable lengths for carrying
electrical energy, particularly in the field of medium and high voltages,
the cable conductors are uncovered at the ends, that is, devoid of the
respective insulating coatings and, if included, the semiconductive
coatings, for the purpose of exposing the conductors thereby allowing
their mutual connection. Subsequently, the area without the insulating
coating is filled with appropriate materials and then covered with an
outer covering so as to restore the required insulating characteristics in
the junction area.
For the purpose, a tubular covering element, hereinafter referred to as a
sleeve, is fitted over the junction area. The sleeve is made of
cross-linked polymeric material consisting of several layers each having
specific electrical features, and as a whole, the sleeve is designed to be
elastically clamped around the surface of the insulating layer of the
connected cables covering the insulating layer itself over a length
thereof in the areas contiguous to the conductor junction.
Therefore, the sleeve is radially expanded and maintained under expanded
conditions until it is put over the cable junction area. After the sleeve
has been put in place, its shrinkage around the cable surface is carried
out so that it exerts a pressure thereon capable of ensuring the necessary
electric requirements.
In order to keep the sleeve under expanded conditions, it can be made of
thermoshrinkable material, i.e., a material which is capable of
maintaining the expansion it has received until its shrinkage by heat is
caused. However, this technique requires the accomplishment of delicate
operations on site for the installation of the sleeve because heating
means, such as free flames, are needed in order to achieve the shrinkage
of the sleeve itself.
Alternatively, the sleeve may be made of an elastic material and fitted
under expanded conditions around a tubular support body which is then
removed after the sleeve has been brought to the intended position around
the cable junction area, which enables the sleeve to elastically shrink
and be clamped around the cable insulating layer.
However, the polymeric materials to be used for the manufacture of sleeves,
in which each layer needs particular electrical features well known in the
field, generally exhibit, on the discontinuance of the mechanical
deformation stress held for a certain period of time, an incomplete
elastic return to the original size, that is, the sleeve has an initial
temporary residual deformation the degree of which depends, among other
things, upon the value of the previously imposed deformation and the
temperature and time of stay in the deformed states. Said residual
deformation decreases as time goes by and tends to become zero after a
certain lapse of time, in the range of some days or months at room
temperature (.ltoreq.30.degree. C.).
Due to the fact that after the sleeve has been fitted on the cable
junction, it is impossible to wait for a time sufficient to achieve the
size recovery which is necessary to the correct clamping of the sleeve,
the problem arises of providing a sleeve which, after being stored under
expanded conditions, can still be efficiently clamped around the cables by
virtue of its elastic features only.
Materials having particular properties of reduced residual deformation
could be used for manufacturing the sleeve, but such materials are of
difficult formulation because the mechanical characteristics required of
them are not normally accompanied by the necessary electrical properties
for the different layers so that it becomes difficult to manufacture a
sleeve wherein all layers have an elastic behavior exhibiting a reduced
residual deformation.
Taking into account the above problem, sleeves of the type described in
said patent application Ser. No. 07/464,370 have been manufactured, in
which the use of a single sleeve size has been provided for covering cable
junctions having different sizes through the employment of covering
elements made at the factory and kept in an expanded condition until they
are applied.
In said patent application, the problem of providing the sleeve with
sufficient expansion to enable it to be fitted over the cable of the
greatest diameter in the group of the intended sizes while at the same
time enabling it to be efficiently clamped also around the cables of the
smallest size in the group has been solved by adopting for the radially
outermost sleeve layer only, a material which exhibits a reduced residual
deformation on discontinuance of the applied expansion stress, which
material is therefore adapted to act on the underlaying layers so that the
whole sleeve can efficiently be clamped around the smallest cables in the
intended range of sizes.
However, in order to be able to perform its function in an efficient
manner, the outer layer must develop an elastic force sufficient to impose
the desired shrinkage to the underlying layers, and therefore, it must
have a particularly high modulus of elasticity and in addition its
thickness must be higher than it would be necessary if only the desired
electrical performance of the layer were involved. Furthermore, the
material forming the outer layer appears to be comparatively less stressed
in the expanded state, relative to the material of the inner layers, for
which the use of materials possessing more reduced mechanical qualities is
dictated.
BRIEF DESCRIPTION OF THE INVENTION
The present invention has, as one object, the providing of a covering
sleeve for electric cable joints which can be prepared at the factory,
expanded on a tubular support and stored as such until the moment of use
and can be employed for differently sized cables while ensuring an
appropriate clamping of the cables without requiring modifications in the
sizes of the different layers as established on the basis of the
electrical performance required thereof, and which also makes possible the
disposing of the material provided with the best mechanical
characteristics in the area subjected to the greatest deformation
stresses.
In accordance with the present invention, a storable covering element for
electric cable joints which can be applied to several cables having
different outer diameters, comprises a tubular support and a sleeve
stretched over it. The sleeve comprises a plurality of coaxial layers, at
least one of which is electrically insulating, made of compounds formed
with cross-linked polymeric materials and is fitted over said tubular
support in a condition of elastic radial expansion. The sleeve layers are
integrally linked or bonded together and can withstand an imposed
expansion maintained for at least 24 months at room temperature. The
compound of the innermost layer exhibits, in the cross-linked state, an
instantaneous residual deformation substantially lower than that of the
layers located externally to it.
Preferably, the compound of the innermost layer at the cross-linked state
exhibits an instantaneous residual deformation lower than 15% in
connection with an imposed expansion of 50%, maintained for at least 24
months at room temperature or, alternatively, for at least 40 days at
65.degree. C.
Conveniently, the compound of the innermost layer has a modulus of
elasticity as great as that of the layers located more externally.
Preferably, the innermost layer has a radial thickness lower than 25% of
the overall thickness of the sleeve and is formed with a compound having a
modulus of elasticity as great as 1.5 times the modulus of the layers
located externally thereof.
In a preferred embodiment, the compound of the innermost layer has a
modulus of elasticity included between 0.5 and 10 MPa and the compounds of
the layers located more externally relative to it have a modulus of
elasticity in the range of 0.03 to 6 MPa. In a more preferred embodiment,
the compound of the innermost layer has a modulus of elasticity included
between 1 and 5 MPa and the compounds of the outer layers have a modulus
of elasticity in the range of 0.5 to 3 MPa.
The sleeve layers are made of elastomeric materials having chemical
affinity therebetween, and preferably are coextruded and/or jointly
cross-linked.
A further object of the present invention is an electric cable joint
comprising an electrical and mechanical connection between the conductors
of two cables and a covering sleeve for the connection itself, fitted over
the connection and in contact with the insulating coatings of the cables
in a condition of elastic radial expansion. The sleeve is formed with
several coaxial layers, at least one of which is an electrically
insulating layer, made of compounds of cross-linked polymeric materials.
The sleeve layers are integrally linked together and under an imposed
expansion maintained for at least 24 months at room temperature, the
compound of the innermost layer exhibits, in the cross-linked state, a
temporary residual deformation substantially lower than that of the layers
located externally to it.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the present invention will be apparent from
the following detailed description of the presently preferred embodiments
thereof, which description should be considered in conjunction with the
accompanying drawings in which:
FIG. 1 is a diagrammatic, axial, sectional view of the junction area of two
cables having the covering sleeve applied thereto;
FIG. 2 is an axial sectional view of the junction area of the cables shown
in FIG. 1 with the covering sleeve being applied on the cables;
FIG. 3 is a cross-sectional view of a sleeve in accordance with the
invention in its unstretched condition;
FIG. 4 is a cross-sectional view of the sleeve shown in FIG. 2 after
stretching or expansion and disposed on the tubular support; and
FIG. 5 is a diagram showing the radial development of the percent expansion
in the sleeve thickness.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
As diagrammatically shown in FIG. 1, the junction between two cables 1, 2
is made by disposing the ends of the cables to be joined in a confronting
coaxial relation, the layers around the conductors 3 of the cables having
been previously removed stepwise so that the conductors 3 are bared over
predetermined lengths thereof.
The bared end portions of conductors 3 are electrically and conductively
connected to each other by a junction means 8.
Said electrical junction means 8, diagrammatically shown in FIGS. 1 and 2,
is known per se, and it may, for example, consist of a welding, a scarfing
element or the like and therefore, will not be herein further described.
After the conductors have been electrically connected to each other, the
space corresponding to the lengths where the insulating layer 4 has been
removed, is filled with a deformable field-control filler 9, which is also
well known in the art. A further conductive means 9a of known type covers
the ends of the covering sleeve 10 and restores the armoring continuity,
such means 9a being conductively connected to the semiconductive layers 5
of the cable and to the semiconductive layer 10c of the covering sleeve.
For covering the junction area, a sleeve 10 is provided and is made of
cross-linked polymeric material. The sleeve 10 is slidably fitted over one
of the cables before it is joined to the other cable by the means 8. Once
the electrical connection operations have been completed through the
junction means 8, and the filler 9 has been applied, the sleeve 10 is
brought over the junction area and released in place so as to form a cover
completely surrounding the uncovered lengths of the insulating layer 4.
For the purpose of being fitted over the junction area and as shown in FIG.
2, the sleeve 10 is mounted on a tubular support body 11 which keeps the
sleeve 10 in a radially expanded condition. The inner diameter of the body
11 is of a size which allows the sleeve itself and the tubular body 11
carrying it to freely slide on the cables, at least on the portion forming
the semiconductive layer 5 or, as shown, the outer sheath 7.
In order to enable the sleeve to be put around the junction, the tubular
body 11, as diagrammatically shown in FIG. 2, is progressively removed
using known techniques, for example, by axial withdrawal, so that the
sleeve can elastically shrink around the cable surfaces, ejecting the air
from the contact area and adhering to said surfaces, while exerting some
pressure thereon in order to ensure a correct distribution of the electric
field in the junction area.
Usually for medium voltage cables (U.sub.m .gtoreq.12 KV) this pressure
must be higher than a minimum value of about 0.1 MPa.
In this case, the sleeve 10 is required to have an inner diameter at rest,
that is in the absence of applied stresses, smaller than the outer
diameter of the cable insulating layer by an amount giving rise to an
elastic deformation of the sleeve corresponding to the desired clamping of
said sleeve on the cables. In other words, the bore of the sleeve must
have a diameter less than the diameter of the insulating layer 4 of the
cables so that when the sleeve 10 engages the layer 4, the sleeve 10 is
still in an elastically deformed condition.
The required degree of difference in diameter for developing a certain
pressure value on the cable depends upon the elastic deformability of the
material forming the sleeve, that is, on its modulus of elasticity E and
on its radial thickness. Therefore, these values must be selected so as to
comply with the desired pressure as above stated.
The structure of a sleeve for medium voltages and of the type adapted to be
applied as above stated and as shown in FIGS. 1, 2 in axial section and in
FIG. 3 in transverse cross-sectional view, consists of several coaxial
layers which, starting from the central bore 12, comprises an inner layer
10a, acting as a voltage divider and made of a material having a high
dielectric constant .epsilon., an intermediate layer 10b made of
insulating material and an outer layer 10c of semiconductive material.
The term "material having a high dielectric constant" means a material
having a dielectric constant .epsilon. determined according to
specifications ASTM D 150 at least equal to 6 and preferably at least
equal to 9, whereas the term "insulating material" means a material having
a dielectric constant .epsilon. according to specifications ASTM D 150
lower than 4 and volume resistivity>10.sup.14 Qcm, determined according to
the specification ASTM D257.
In order to enable the sleeve to be fitted over the tubular support body
11, it must be radially expanded so that its bore 12 having a diameter
D.sub.1 at rest, as shown in FIG. 3, will reach the diameter D.sub.2 as
shown in FIG. 4.
The diagram shown in FIG. 5 represents the development of the percent
expansion state in the sleeve thickness at the various radial positions r,
emphasizing the expansion values at the inner radius r.sub.i, at radius
r.sub..epsilon. of the outer surface of the layer 10a having high
.epsilon., at radius r.sub..epsilon. of the outer surface of the
insulating layer 10b and at the outer radius r.sub.e of the sleeve 10.
The diagram shown in FIG. 5 has been determined taking into account an
expansion of the sleeve from a starting diameter D.sub.1 =17 mm to a
diameter D.sub.2 =40 mm, for a sleeve having an overall wall thickness of
15 mm.
As viewed in the diagram, the development of the percent expansion is of
the hyperbolic type. Therefore, it is very high for the areas close to the
inner radius r.sub.i of the sleeve, whereas it is remarkably reduced
proceeding outwardly.
The materials used for manufacturing the sleeves are cross-linked polymeric
materials, consisting of compounds selected on the basis of the required
electric characteristics for each layer.
These materials in general do not have a perfectly elastic behavior. In
fact, when they are submitted to deformation they exhibit a certain degree
of temporary residual deformation which becomes increasingly higher with
the increasing of the imposed expansion.
For example, if some tubular sleeves are expanded starting from a given
inner diameter and are maintained in the expanded state for a certain
period of time, upon release they elastically come back to a greater
diameter than the starting one, thereby retaining a deformation which is
not immediately eliminated, particularly if the deformed condition has
been maintained for a long time, as in the case of the sleeves expanded at
the factory and stored under an expanded condition until the moment of
use, e.g. for some months. Actually, the residual deformation will
decrease either partly or to zero, but it takes very long periods of time
for such decrease, i.e. weeks or months.
Since it is desirable to be able to mount the sleeve on the tubular bodies
at the factory, under supervised conditions, and keep them in an expanded
state on said supports until the moment of use, normally for a period of
some months, the problem of the residual deformation is particularly
important because this residual deformation can impair the desired
clamping of the sleeve around the cables.
According to the invention, in order to enable the sleeve assembly to be
applied to the cable junction in compliance with the desired
radial-clamping conditions, the innermost layer 10a of the sleeve is made
of a material having a reduced degree of residual deformation, lower than
that of the overlying layers.
In this way, the inner layer when released after expansion, by virtue of
its reduced residual deformation, is capable of elastically resuming a
diameter slightly higher than the starting one, which therefore ensures a
good clamping on the cables. In addition, it must carry out a pulling
action on the layers externally overlying it in order to make the assembly
resume its original size, counteracting the residual deformation present
in said outer layers.
In particular, the innermost layer, as soon as the tubular support keeping
it expanded has been removed, is subjected to a spring back or return
force, the amount of which depends on its modulus of elasticity, its
thickness and the value of the relative deformation imposed thereon.
When the sleeve 10 is fitted over the support 11 under expanded conditions,
the inner layer 10a is in the highest state of deformation with respect to
the other sleeve layers. Therefore, upon release, the spring back force
acting thereon is the highest of the various layers, the other conditions
being equal.
Therefore, the use in this layer 10a of a compound having a low temporary
residual deformation and capable of elastically shrinking until the
central bore 12 reaches the diameter corresponding to the desired value
for being clamped around the cables allows the desired result to be
achieved, that is, the return of the whole sleeve to a diameter close to
the starting one, causing the other layers to shrink although the inner
layer has a relatively reduced thickness and the overlying layers exhibit
a higher residual deformation.
In order to achieve the above result, causing a high return force for the
inner layer 10a, that is, the one causing the shrinkage of the whole
sleeve to the desired degree, the inner layer 10a should preferably have a
modulus of elasticity not lower than that of the other layers and, more
preferably, assuming that the other layers 10b and 10c have typical sizes
based on the required performance from the electrical point of view, the
modulus of elasticity of the layer 10a should be higher than 1.5 times the
modulus of elasticity of said other layers.
In particular, the modulus of elasticity of the compounds forming the outer
layers 10b, 10c can be between 0.03 and 6 MPa, whereas the modulus of the
compound forming the inner layer 10a can be in the range of 0.5 to 10 MPa,
but at least equal to the modulus of elasticity of the layers 10b and 10c.
Preferably, the compound used for the innermost layer has a modulus of
elasticity in the range of 1 to 5 MPa and the compounds of the outer
layers have a modulus of elasticity between 0.5 and 3 MPa.
As regards the outer layers 10b and 10c, they are in turn submitted to an
expansion state which is relatively smaller on the support 11 and
therefore the residual deformation of said layers, since it increasingly
grows with the increasing of the imposed deformation, is at all events
within values which enable it to be resumed by the returning action
carried out by the inner layer 10a.
In order that the inner layer 10a of the sleeve should be able to exert its
returning action, it must be integrally linked to the external layer
immediately overlying it and the latter in turn must be linked to the
layer 10c in the same manner so that the layer 10a will be capable of
transmitting the necessary tractive action to the outer layers 10b and 10c
thereby enabling the clamping of the sleeve around the cables. This
condition is easily complied with in the case in which the layers have
chemical affinity with respect to one another and are coextruded and/or
jointly cross-linked to form the sleeve.
Preferably, when sleeves are sized so as to be used for applications in the
field of medium and high voltages, in which the innermost layer has a
thickness lower than 25% of the overall wall thickness of the sleeve and
it is wished to apply the sleeve to cables of several different diameters
selected from a group in which the diameter of the greatest cable is 1.6
times larger than the diameter of the smallest one, the material forming
the inner layer 10a is required to have an instantaneous residual
deformation lower than 15%, taking into account an imposed deformation of
50% held for a period of 40 days at 65.degree. C. and determined on a flat
test piece following the procedure stated by the specification UNI
7321-74.
The stated time and temperature conditions are substantially equivalent to
a two years' stay at room temperature, and therefore, such conditions
simulate, in an accelerated manner, the actual conditions in which a
sleeve expanded at the factory can be at the moment of use and after a
storage period, thereby constituting an appropriate reference value for
evaluating the elastic behavior of the material.
By way of example, a sleeve in accordance with the invention, adapted to be
used on medium voltage cable joints, in a cable size range comprised
between a diameter of 20 mm and a diameter of 32 mm for the insulating
coating, taking into account the choice of sizes based on the requested
electrical performance, requires a thickness of about 2 mm for the
innermost layer 10a, a thickness of 7 to 10 mm for the intermediate
insulating layer 10b and a thickness of 1 to 3 mm for the outermost layer
10c.
The diameter at rest D.sub.1 of the central bore 12 in the sleeve 10 is for
example 17 mm and the sleeve expansion on the tubular support 11 makes the
sleeve reach a diameter of 40 mm.
An example of a compound made of polymeric material susceptible of
cross-linking having the stated characteristics of deformability and
strength, to be used for manufacturing the inner layer 10a of a sleeve in
accordance with the above example has the following composition expressed
in parts by weight:
______________________________________
ethylene-propylene-diene terpolymer,
100
for example the one known on the
market under the name DUTRAL TER-
048 sold by DUTRAL S.p.A.
zinc oxide 5
lead tetroxide (Pb.sub.3 O.sub.4)
5
conductive carbon black, for example
140
the one known on the market under the
name HUBER N 990 sold by DEGUSSA S.p.A.
paraffin plasticizer 40
poly-1,2-dihydro-2,2,4-trimethyl
1.5
quinoline
stearic acid 1
triallyl cyanidate 1.5
40% active cumene hydroperoxide
10
______________________________________
The physical characteristics of the sleeve layer 10a made with said
compound, after cross-linking, are as follows:
______________________________________
tensile breaking strength
7 MPa
ultimate percent pulling elongation, at
610%
room temperature (20.degree. C.)
modulus of elasticity E 3 MPa
temporary residual deformation at
10%
the imposed elongation of 50% determined
according to UNI specifications 7321-74
on a flat test piece, at 65.degree. C. and after a
lapse of time of 960 hours
dielectric constant .epsilon. determined
15
according to ASTM specifications D150
volume resistivity determined according
10.sup.10 Ohm .times. cm
to ASTM specifications D257
______________________________________
An example of a compound made of a polymeric material susceptible of
cross-linking having the stated characteristics, to be used for making the
insulating layer 10b in the example illustrated has the following
composition expressed in parts by weight:
______________________________________
ethylene-propylene copolymer, for
100
example the one known on the market
under the name DUTRAL CO-054 SOLD BY
DUTRAL S.p.A.
zinc oxide 5
lead tetroxide (Pb.sub.3 O.sub.4)
5
stearic acid 1
calcined kaolin having a surface
70
treatment with trimethoxyethoxy
vinylsilane
trimethoxyethoxy vinylsilane
1
paraffin plasticizer 18
poly-1,2-dihydro-2,2,4-trimethyl
1.5
quinoline
mercaptobenzoimidazone 2
triallyl cyanidate 1.5
40% active-bis-(terbutyl 5
peroxy) m p diisopropylbenzene
______________________________________
The physical characteristics of the sleeve layer 10b made with said
compound, after cross-linking, are as follows:
______________________________________
tensile breaking strength
7 MPa
ultimate percent pulling elongation, at
560%
room temperature (20.degree. C.)
modulus of elasticity E 1.5 MPa
temporary residual deformation at
28%
the imposed elongation of 50% determined
according to UNI specifications 7321-74
on a flat test piece, at 65.degree. C. and after a
lapse of time of 960 hours
dielectric constant .epsilon. determined
2.8
according to ASTM specifications D150
volume resistivity determined according
10.sup.15 Ohm .times. cm
to ASTM specifications D257
______________________________________
For the outermost semiconductive layer 10c, an appropriate compound has the
following composition expressed in parts by weight:
______________________________________
ethylene-propylene-diene terpolymer,
100
for example the one known on the
market under the name DUTRAL TER-
054 sold by DUTRAL S.p.A.
zinc oxide 5
conductive carbon black, for example
80
the one known on the market under the
name VULCAN P sold by CABOT S.p.A.
paraffin plasticizer 35
poly-1,2-dihydro-2,2,4-trimethyl
1.5
quinoline
stearic acid 1
triallyl cyanidate 1.5
40% active cumene hydroperoxide
7.5
______________________________________
The physical characteristics of the sleeve layer made with said compound,
after cross-linking, are as follows:
______________________________________
tensile breaking strength
11 MPa
ultimate percent pulling elongation, at
520%
room temperature (20.degree. C.)
modulus of elasticity E 1.5 MPa
temporary residual deformation at
31%
the imposed elongation of 50% determined
according to UNI specifications 7321-74
on a flat test piece, at 65.degree. C. and after a
lapse of time of 960 hours
volume resistivity determined according
180 Ohm .times. cm
to ASTM specifications D257
______________________________________
A sleeve having the described structure can therefore be expanded over the
tubular support 11 until it reaches an inner diameter of 40 mm and held at
the expanded state until the moment of use.
Under these conditions, the inner layer 10a, having a modulus of elasticity
E which is twice the modulus of the other outer layers and exhibiting a
temporary residual deformation lower than 15% in connection with an
expansion of 135%, is capable of ensuring both its elastic return to a
diameter of 19 mm so as to accomplish the desired pressure with the cables
and a returning action on the other outer layers thereby overcoming the
tendency of the latter to stay in a state of residual deformation and
bringing about the return of the assembly to the desired diameter.
Therefore, with a sleeve in accordance with the invention, it is possible
to obtain an appropriate clamping pressure on cables of a wide range of
diameters by using a material having a reduced residual deformation for
only one of the sleeve layers and without being obliged, for reasons of
mechanical behavior, to modify the choice of the thickness sizes for the
sleeve layers. Therefore, such sizes can be determined depending on the
electrical requirements only. In addition, the invention enables an
optimal exploitation of the elastic properties of the materials used, due
to the fact that the material having the best elastic features, which are
usually associated with the best strength properties, is located at the
position in the sleeve which is subject to the maximum stresses, whereas
in less stressed areas, materials having reduced mechanical qualities can
be used.
Although preferred embodiments of the present invention have been described
and illustrated, it will be apparent to those skilled in the art that
various modifications may be made without departing from the principles of
the invention.
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